This invention relates to a process for the transmission of selective addresses and/or emergency messages in shortwave radio networks comprising several stations containing a transmitter and/or a receiver by using a ringing signal consisting of a synchronization and an address signal transmitted from a transmitter.
Shortwave connections mainly use the propagation of spacewaves reflected from the ionosphere for transmitting messages over long distances. In spite of the shortcomings of the transmission channel for a spacewave connection, such as noise interferences in the channel, time variable, dispersive channel response and the presence of selective interference, this type of transmission has recently gained significant importance by virtue of new microprocessing techniques and the low cost compared with satellites.
Today, transmissions are conventionally carried out by the single-sideband technique for economic utilization of the frequency reserve. The frequency of the signal from the audio frequency band (300 Hz to 3.4 kHz) is translated into a selected high frequency (i.e., HF) band at the transmitter end and the reverse operation is carried out by the HF receiver. The received signal is transmitted to the demodulator and decoder circuits in the audio frequency (i.e., AF) range. The HF receivers have automatic gain controls wherein the total output or voltage forms the controlled variable within the selected bandwidth of the receiver channel. Different noise levels and user signal levels are established within wide limits at the output, depending on the spectral occupation of the user signal and interference signal. Selective interferences having a greater signal energy than the user signal are frequently encountered and the channel is then in most cases regarded as occupied.
In a selective calling network, various stations should be capable of being activated singly or by use of a collective code. The selective calling transmitters and receivers of the individual stations are accommodated in their modulator and demodulator block. The calling signals are composed of a set of suitable amplitude-time functions which the individual receivers can recognize in the channel noise and distinguish from one another. Even when the quality of transmission is poor, no false stations should be activated and the required stations should always be activated.
The conventional pilot sound transmissions used at present are not capable of fulfilling these requirements because the probability of faulty synchronization increases with the presence of certain interferences.
It is known to use a relatively complicated system of equipment in addition to the transmitter-receiver parts of the stations for determining which channel is free and undisturbed by neighboring transmitters and for determining the momentary conditions of propagation over the ionosphere ("Assessing HF Propagation Conditions in Real Time", Defense Electronics, May 1980, pages 21, 22). A fully automatic setup of a shortwave communication cannot be obtained with this system. When interference (jammers) are deliberately produced (electronic counter measurement--ECM), previous channel analysis is of little use since the interference always occurs instantaneously in the channel in use.
European Patent Application No. 87 105 972.1 (=EP-A-0 243 885) of 23.04.1987 describes a process by which fully automatic shortwave connections can be set up between individual stations and only the desired stations are activated even when the quality of transmission is poor. This is achieved by using a very pronounced synchronization signal which is adapted to the transmission channel and is composed of narrow band marking and space signals which form the part signals of a diversity pair.